Fumed silica is often used in coating compositions. The presence of isolated hydroxyl groups on the surface of fumed silica particles often provide desirable Theological properties, such as thixotropy, viscosity, and sag resistance, to coating compositions, such as compositions in the form of aqueous dispersions. Coating compositions in the form of aqueous dispersions are desirable in many applications because they often alleviate some of the environmental hazards associated with organic solvent based coating compositions.
Fumed silica is widely available commercially. Exemplary products include CAB-O-SIL® untreated amorphous fumed silica, commercially available from Cabot Corp., and Aerosil® fumed silica, commercially available from Degussa AG. The term “fumed silica” refers to materials manufactured by high temperature hydrolysis of a volatile silane compound, such as a chlorosilane, in a hydrogen/oxygen flame.
One disadvantage associated with existing commercial processes for manufacturing fumed silica is that they utilize, as indicated, liquid volatile silane materials. These compounds can be relatively expensive as compared to readily available solid silica powders, such as silica flour, silica fume and even synthetic precipitated silica. As a result, it would be advantageous to provide a commercially viable process and/or apparatus for the production of ultrafine silica particles from solid silica powders. Moreover, it would be desirable to provide ultrafine silica particles produced from solid silica powders that exhibit rheology properties similar to fumed silica particles, such that the ultrafine silica particles can effectively at least partially replace fumed silica particles in coating compositions.